A technique for improving corrosion resistance and abrasion resistance of a metallic material by coating the surface of the metallic material by means of an in-liquid electric discharge machining has been known. One such technique is described below.
For example, the following method is disclosed (refer to the patent literature 1). In the method, an electrode formed by compression molding a mixture of WC (tungsten carbide) powder and Co powder is used to deposit the material of the electrode on the work by in-liquid pulsed electric discharge, then a re-melting discharge machining is carried out using another electrode (for example, a copper electrode or a graphite electrode) to obtain a film with higher hardness and higher adhesion. In other words, WC—Co is deposited on the work (base metal S50C that is a kind of steel prescribed by Japanese Industrial Standard JIS G 4051) using an electrode of a green compact mixture of WC—Co by performing the in-liquid discharge machining (primary machining), subsequently re-melting machining (secondary machining) is performed using an electrode, such as copper electrode, that is not consumed very rapidly. As a result, the deposited structure had a low hardness (Vickers hardness Hv) of about Hv=1410 and there were a lot of voids at the end of the primary machining; however, the voids in the coat disappeared and the hardness improved to Hv=1750 after the re-melting machining was performed as the secondary machining. Thus, a hard coat with strong adhesion to the work, which is steel, can be obtained when the above-mentioned method is used.
However, with the above-mentioned method, it is difficult to form a coat having strong adhesion to the surface of the sintered material such as cemented carbide as a work. In this connection, it was confirmed in the research performed by the inventors of the present invention that it was possible to form a sturdy hard coat on the surface of the metallic work without performing the step of re-melting, if an electric discharge is generated between the work and an electrode of such material as Ti that forms hard carbide. The sturdy hard coat is formed due to the generation of TiC as a result of reaction between the material of the electrode, which is attrited by electric discharge, and carbon C that is a component of the dielectric fluid.
Further, a technique is disclosed in which an electric discharge is generated between an electrode of green compact metallic hydride such as TiH2 (Titanium Hydride) and a work to form more speedily a hard coat having higher adhesion than when a material such as Ti is used (refer to the patent literature 2). Further, a technique is disclosed to speedily form a hard coat having various characteristics such as high hardness and high abrasion resistance by generating an electric discharge between a work and an electrode of green compact composed of hydride such as TiH2 (Titanium Hydride) with which different metal or ceramic are mixed.
Moreover, there is a disclosure of another technique that teaches that it is possible to produce a sturdier electrode only by performing a preliminary sintering (refer to the patent literature 3). Namely, in manufacturing of an electrode composed of a mixture of WC powder and Co powder, the green compact may be manufactured by merely mixing WC powder with Co powder and by compression molding; however, if the compression molding is performed after wax is added to the powders, compression molding the green compact becomes easier and more efficient. However, when the wax is added and if a large amount of the wax remains in the electrode, the electric resistance of the electrode increases because the wax is dielectric, resulting in a poor electric discharge performance. Therefore, the wax is removed from the electrode by heating the green compact electrode in a vacuum furnace. In removing the wax, it is necessary to keep the heating temperature higher than the melting point of the wax and lower than the temperature at which the wax decomposes and turns into soot; because the wax will not get removed from the electrode if the heating temperature is too low, and the purity of the electrode degrades if the wax turns into soot because the heating temperature is too high. Further, the green compact in the vacuum furnace is heated by a high-frequency coil and the like so that the green compact has enough strength so as to withstand machining while preventing the green compact from becoming too hard (this is called a preliminary sintering state), in other words, the green compact is heated until the compact becomes as hard as, for example, a chalk. Bonding among the carbides at the contact parts proceeds interactively, in the preliminary sintering state; however, bonding strength is weak because the sintering temperature is lower than the temperature required for the standard sintering. It is found that it is possible to form a closely-packed homogeneous coat if the electric discharge surface treatment is performed using the electrode obtained in this manner.
Each of the above-mentioned conventional art has features in hardness and adhesion of the coat, abrasion resistance and swiftness of forming the coat, and, density and homogeneity of the coat; however, with regard to the thickness of the coat, no conventional art is sufficient, thus leaving scope for improvement.
So called welding and thermal spray coating are known as general techniques to build up a thick coat. Welding (refers to a build up welding here) is a technique of letting the material of the welding rod melt and adhere to the work by an electric discharge between the work and the welding rod. The thermal spray coating is a technique of melting the metallic material and spraying the melted material onto the work to form a coat. Since either method is a manual labor requiring skills, which makes it difficult to establish a continuous production line, the both methods have a drawback of having a high production cost. Moreover, especially as welding is a method in which heat enters the work convergently, when dealing which thin materials or brittle materials such as single crystal alloy and directional control alloy such as unidirectionally solidified alloy, cracks are easily produced and lower the yield.
The Patent Literature 1
Japanese Patent Application Laid-Open No. H5-148615
The Patent Literature 2
Japanese Patent Application Laid-Open No. H9-192937
The Patent Literature 3
Japanese Patent No. 3227454
Non-Patent Literature 1
“Formation of Thick Layer by Electrical Discharge Coating (EDC)”, Goto Akihiro et al., Mold Technique, (1999), Nikkan Kougyou Shinbunsha.
The emphasis has been placed on forming a hard coat in the conventional electric discharge surface treatments; therefore, the main materials of the electrode are hard ceramic materials or the material that forms hard carbide by a chemical reaction with C (carbon) that is a component of the oil in the dielectric fluid, due to the electric discharge energy. However, hard materials generally have a high melting point and a low heat conductivity characteristic. Therefore, although it is possible to obtain a closely packed coat of a thickness of the order of 10 micrometers (μm), it is very difficult to obtain a closely packed coat of a few 100 μm or thicker.
Although it is described in the literature (see non-patent literature 1) based on a study by the inventors of the present invention that about a 3-millimeter-thick coat was obtained using an electrode of WC—Co (9:1), the technique described is difficult to put into practical use, because it has such problems as reproduction is difficult due to unstable coat formation, the coat is brittle having a lot of voids, and the coat is so weak as that it is removed if scraped with a piece of metal even though the coat apparently has metallic luster and looks closely packed.
Further, regarding the above-described welding and the thermal spray coating to build up the coat, both the techniques have problems because they require a lot of manual work which results in higher production cost because of difficulty in building a line production plant, and lower yield because of generation of welding cracks.
It is an object of the present invention to provide an electrode for electric discharge surface treatment, a method of electric discharge surface treatment, and an apparatus for electric discharge surface treatment to form a thick coat, which was difficult for the coating formed by the conventional in-liquid pulsed electric discharge treatment. It is another object of the present invention to provide an electrode for electric discharge surface treatment, a method of electric discharge surface treatment, and an apparatus for electric discharge surface treatment to form a high quality coat in the coating by the in-liquid pulsed electric discharge treatment.